The invention relates to a method for production of a device for thermal detection of radiation comprising at least one active microbolometer and at least one passive microbolometer, each comprising a suspended membrane performing the function of radiation absorber, thermometer and electrical connection, the active and passive microbolometers being formed simultaneously on a single support substrate and a reflective screen being formed on the whole of the device, and then eliminated opposite the locations of the active microbolometers.
The invention also relates to a passive microbolometer produced by such a method.
1. State of the Art
In general manner, a microbolometer with a microbridge comprises a suspended membrane supported by anchoring elements on a support substrate. The membrane presents three functions, i.e. incident radiation absorption by means of an absorbing element, transformation of calories into resistance variation by means of a thermometric element, and electrical connections with the support substrate by means of one or more electrodes.
These three functions can be performed by three separate elements. The absorbing element, which heats due to the effect of an incident radiation, transmits the heat to a thermometric element, the temperature rise of which element is preferably measured electronically by an electronic measuring circuit external to the microbolometer. Electrical connections of the membrane with the support substrate are achieved for example by means of electrodes. The absorbing element is thus designed to convert an incident luminous flux, for example photons, into a heat flux. The heat flux induces a temperature variation of the thermometric element, which converts the temperature variations into electrical signals. The support substrate, above which the membrane is suspended, constitutes the cold point of the microbolometer and contains the electronic measuring circuit that uses the electrical signals.
In certain cases, these three functions can be performed by two elements only. For example, a bolometric material can perform the function both of absorbing element and of thermometric element, electrical connection with the support then being achieved by the electrodes connected to the thermometric element.
In another alternative embodiment, the electrodes can at the same time perform the function both of electrical connection and of absorbing element. The bolometric material then constitutes the thermometric element only.
The electrodes, for example in the form of a coil, can also perform the function both of electrical connection and of thermometric element, the absorbing element being a separate element.
In FIG. 1, the microbolometer 1 comprises a membrane suspended on a support substrate 3 by means of two anchoring elements 4, also forming a thermal link between the membrane and the substrate 3. The membrane comprises at least one absorbing element 2 supporting a thermometric element 5, the temperature variation of which element is measured by means of electrodes (not shown). The support substrate 3 comprises an electronic measuring circuit (not shown) to use the measurement made by the microbolometer 1. The sensitivity of measurement can be improved by introducing insulating arms 6 between the support substrate 3 and the membrane to limit the heat losses of the membrane and to consequently preserve heating thereof.
The thermometric element 5 can be of resistive type. It is then the variation of the resistance and/or of the impedance of the thermometric element 5 that is measured. For example, the thermometric element 5 can be formed by a bolometric material in contact with the electrode(s), which, due to a special configuration, for example in the form of a coil, perform both the role of absorbing element and of electrical connection. An incident radiation absorbed by the microbolometer 1 then causes a temperature increase of the absorber 2, which results in a variation of the electrical resistance of the thermometric element 5. This resistance variation is measured at the terminals of the electrodes, which are preferably securedly affixed to the anchoring elements 4.
Efficient operation requires three main conditions to be met as far as the microbolometer 1 is concerned: a low calorific mass, a good thermal insulation of the membrane from the support substrate 3 and a good sensitivity of the conversion effect of the heat rise into an electrical signal. The first two conditions are achieved by implementing thin layers to achieve the microbolometer 1.
FIG. 2 illustrates the reading principle of a microbolometer-based detection device. The device comprises a measuring microbolometer 7, or active microbolometer, that absorbs an incident radiation 8, for example infrared rays. The variation of the resistance of the microbolometer 7 is representative of the value of this radiation. Current reading is frequently used to make this measurement. The current, on output from the microbolometer 7, comprises a variable fraction and an unvarying fraction. The detector in fact operates in relative manner, i.e. it detects a continuous unvarying background signal which may hamper measurement of the useful variable signal, which is in general small compared with this background signal. This unvarying fraction of the current therefore has to be eliminated to obtain optimal measurement of the radiation value.
To increase the reading sensitivity, the unvarying fraction of the current is preferably branched off to a derivation branch so that only the variable part of the current is sent to an integrator 9. In terms of electronics, the element acting as derivation branch must not be too noisy so as not to generate too much disturbance. For this, the derivation branch is achieved by means of a forward-biased resistor of sufficiently high value. A conventional solution consists in using a passive microbolometer as derivation branch, i.e. a microbolometer that does not detect radiation.
The derivation branch therefore comprises, as represented in FIG. 2, a derivation microbolometer 10, which is made blind by a protective screen 11 placed between the radiation 8 and the microbolometer 10. The microbolometer 10 is thus transformed into a passive microbolometer which does not absorb any radiation and acts as reference.
The efficiency of the detection device is therefore also linked to the characteristics of the passive microbolometer 10, which has to be totally blind and advantageously present a minimal heat resistance.
Other detection devices use a bridge arrangement comprising two microbolometers one of which is made passive by fitting a protective screen between the radiation and this microbolometer (EP-A-0892257 and EP-A-0566156).
Placing a protective screen in front of the microbolometer causes problems as far as manufacturing is concerned.
2. Object of the Invention
The object of the invention is to remedy these shortcomings and to achieve a passive microbolometer, manufacture of the protective screen whereof is integrated in the manufacturing process of the passive microbolometer.
According to the invention, this object is achieved by the appended claims and more particularly by the fact that, the membrane comprising a thermometric element and a radiation-absorbing element performing the electrical connections, the passive microbolometer is formed on the reflective screen which comprises at least one metallic layer in contact with the absorbing element of the membrane.